1. Field of the Invention
The present invention relates to a communication method using directional beam and a radio base station conducting radio communication according to a sector-cell configuration of Code Division Multiple Access (CDMA) scheme adopting spectral spreading. More specifically, the present invention relates to a communication method and a radio base station performing transmission/reception of the directional beam using a plurality of antennas.
2. Description of the Related Art
Conventionally, as a mobile communication system, a cellular system covering a service area by deploying a plurality of radio base stations according to cells is used. Further, a sector-cell configuration, which divides a cell into multiple sectors, is used. A service area is then configured by deploying a base station antenna in respective sectors. For the mobile communication system, normally a three (3) or six (6) sector-cell configuration is adopted as shown in FIGS. 1A and 1B.
Moreover, it is known that transmission/reception of a directional beam using an adaptive antenna array reduces interference from other users (mobile station). The transmission/reception of the directional beam is performed using a plurality of antennas to synthesize signals received at the antennas with an appropriate weight so that interference from the other users can be reduced. In DS-CDMA (Direct Sequence-CDMA) scheme, reception method according to Coherent Adaptive Antenna Array Diversity (CAAAD) using a pilot symbol has been proposed.
For a downlink, a transmission method transmitting the directional beam has also been proposed. In the transmission method, a directional beam is transmitted after compensation for signal amplitude and phase variations occurring on a radio link is performed to a directional beam pattern formed on an uplink. The interference from the other users located in the same sector and other sectors can be reduced by transmitting the directional beam to the downlink.
A dedicated channel for transmission of information data of respective users and a common channel for transmission of control data that is common to all the users are available for the downlink. FIGS. 2A and 2B shows typical examples of the directional beam pattern for the dedicated channel and the common channel at the radio base station performing transmission/reception of the directional beam using the adaptive antenna array.
As shown in FIG. 2A, the interference between users can be reduced by transmitting the directional beam for the dedicated channel of each user using a beam pattern BP21 and BP22, in which coverage is narrowed. On the other hand, as shown in FIG. 2B, a non-directional beam pattern BP23 is used for the common channel in order for all the users located in the sector to receive the control data.
However, in the transmission method using the directional beam described above, the adaptive antenna array has to optimize an antenna weight (weight to be applied to a signal inputted to each antenna) in order to reduce interference by forming a directional beam towards each user appropriately.
Further, in the above method, the antenna weight has to be updated frequently according to a movement of mobile stations MS (both an interfered MS and an interfering MS) and variations in propagation conditions (e.g., a movement of a reflecting object) to maintain transmission of the directional beam towards the mobile station MS. Moreover, the antenna weight is updated at the radio base station using LMS (Least Mean Square) or RLS (Recursive Mean Square) algorithms based on the pilot symbol received from each MS in the conventional method.
In a case where the mobile station MS always transmits an up signal towards the radio base station, e.g., AMR (voice) used under a circuit exchange method, the radio base station can extract required information to update the antenna weight from the pilot symbol. Accordingly, the radio base station can form an appropriate directional beam towards the mobile station MS and track the mobile station MS without difficulty.
However, in a case where the mobile station MS transmits the up signal intermittently (e.g., transmission of packet data or a signal arrived at the radio base station is weak), the radio base station receives the up signal intermittently. This causes occurrence of a period where the radio base station receives no pilot symbols from the mobile station MS. Therefore, the radio base station cannot update the antenna weight and direction of the beam that is determined by the antenna weight during the period.
Specifically, as shown in FIG. 3, if a mobile station 2 moves outside the coverage of the directional beam BP23 (a position shown as notation 2′ in FIG. 3) during a period where a radio base station 1 receives no pilot symbols, a radio link between the mobile station 2 and the radio base station 1 is disconnected. Especially, it is assumed that the mobile station 2 mainly performs data reception during packet data communication, and this causes an interval of the up signal transmitted by the mobile station 2 to be lengthened. Therefore, it is highly possible that the radio link would be disconnected under such situation.